In the past, when measuring a weight with an accuracy of about 0.3 mg, it has been a practice to set an electronic scale in a wind-shielding box to weigh the object.
However, when weighing a very small amount with a good accuracy in a production line of a drug etc., changes in temperature, wind, vibration, magnetic lines of force generated from the control equipment, static electricity, etc. cause a detrimental effect on the measurement accuracy of the electronic scale. Among these, the detrimental effects due to static electricity are extremely hard to prevent or eliminate.
As an apparatus for preventing or removing static electricity, there are known anti-static rods, anti-static brushes, static electricity ion generators, etc. for causing static electricity produced to be discharged to the ground. Further, the method of using a conductive film, conductive coating, etc. to form an electrodischarge circuit is also known. Here, the inventors of this case tried applying these means to a weighing apparatus or devising an effective method of application of these means, but could not achieve an accuracy of about 0.3 mg.
On the other hand, the method of covering a measured object on a weighing pan by a cap made of a conductor so as to cut the electric field between the measured object and outside is also known.
For example, Japanese Examined Patent Publication (Kokoku) No. 8-1394 or Japanese Patent No. 2612905 discloses to prevent the detrimental effect of static electricity when weighing a vial, ampoule, or other drug container by an electronic scale in a process for production of a drug by covering the measured object with a cap made of a conductor.
The inventors of the present case tried applying this method as well, but found that an electrical interaction occurred between the measured object and the conductor cap and were not able to achieve an accuracy of about 0.3 mg in the same way as other conventional methods. Further, they learned that there are the problems that an operation for covering each measured object with a cap at the time of weighing is required, that this is poor in work efficiency and becomes an obstacle in raising the speed of the production line, and that the apparatus as a whole also has to become complicated in mechanism.
Further, as other prior art, Japanese Unexamined Patent Publication (Kokai) No. 4-13933 discloses a scale with a leveler wherein a shield plate is attached to the mechanism of the scale near to the leveler so as to prevent static electricity from having an effect on the leveler.
According to the above-mentioned prior art, when weighing an easily chargeable measured object, it was not possible to eliminate the effects due to static electricity and achieve an accuracy of 0.3 mg so. Further, in the process of production of a drug etc., it was difficult to continuously weigh any chargeable containers or a chargeable container in which powdered or liquid medicines are filled with a good work efficiency.